const int led1 = 2;
const int led2 = 3;
const int led3 = 4;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.println('Enter command');

  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(led3, OUTPUT);

}

void loop() {
  // put your main code here, to run repeatedly:
  if (Serial.available() > 0) {

  String command = Serial.readString();
  command.trim();

  if (command == "L1_ON") {
  digitalWrite(led1, HIGH);
  }

  else if (command == "L1_OFF") {
  digitalWrite(led1, LOW);
  }

  else if (command == "L2_ON") {
  digitalWrite(led1, HIGH);
  }

  else if (command == "L2_OFF") {
  digitalWrite(led1, LOW);
  }

  else if (command == "L3_ON") {
  digitalWrite(led1, HIGH);
  }

  else if (command == "L3_OFF") {
  digitalWrite(led1, LOW);
  }

  else {

    Serial.println('Typing error');
  }
 }
}